Evaluating time-reversed speech and signal-correlated noise as auditory baselines for isolating speech-specific processing using fNIRS

Abstract

Evidence using well-established imaging techniques, such as functional magnetic resonance imaging and electrocorticography, suggest that speech-specific cortical responses can be functionally localised by contrasting speech responses with an auditory baseline stimulus, such as time-reversed (TR) speech or signal-correlated noise (SCN). Furthermore, these studies suggest that SCN is a more effective baseline than TR speech. Functional near-infrared spectroscopy (fNIRS) is a relatively novel, optically-based imaging technique with features that make it ideal for investigating speech and language function in paediatric populations. However, it is not known which baseline is best at isolating speech activation when imaging using fNIRS. We presented normal speech, TR speech and SCN in an event-related format to 25 normally-hearing children aged 6–12 years. Brain activity was measured across frontal and temporal brain areas in both cerebral hemispheres whilst children passively listened to the auditory stimuli. In all three conditions, significant activation was observed bilaterally in channels targeting superior temporal regions when stimuli were contrasted against silence. Unlike previous findings in infants, we found no significant activation in the region of interest over superior temporal cortex in school-age children when normal speech was contrasted against either TR speech or SCN. Although no statistically significant lateralisation effects were observed in the region of interest, a left-sided channel targeting posterior temporal regions showed significant activity in response to normal speech only, and was investigated further. Significantly greater activation was observed in this left posterior channel compared to the corresponding channel on the right side under the normal speech vs SCN contrast only. Our findings suggest that neither TR speech nor SCN are suitable auditory baselines for functionally isolating speech-specific processing in an experimental set up involving fNIRS with 6–12 year old children.

The Benefit of Cross-Modal Reorganization on Speech Perception in Pediatric Cochlear Implant Recipients Revealed Using Functional Near-Infrared Spectroscopy

“…Signal-correlated noise (SCN) formed the second auditory condition. SCN is a non-speech signal which is modulated but is unintelligible, and has been used previously in language studies involving neuroimaging (e.g., Stoppelman et al, 2013 ; Brown et al, 2014 ; Mushtaq et al, 2019 ). The third auditory stimulus was steady speech-shaped noise (SSSN), an unmodulated equivalent of SCN.…”

“…Analyses of fNIRS measurements were conducted in MATLAB in conjunction with functions from the HOMER2 package (Huppert et al, 2009 ) and custom scripts developed in our laboratory and previously used in our work (Dewey and Hartley, 2015 ; Wiggins et al, 2016 ; Anderson et al, 2017 , 2019 ; Wijayasiri et al, 2017 ; Lawrence et al, 2018 ; Mushtaq et al, 2019 ). Channels with poor optode-scalp contact were removed using the scalp coupling index (SCI) technique by Pollonini et al ( 2014 ).…”

“…Specifically, since the perception of speech requires the brain to extract linguistic information from acoustic speech signals (Purves et al, 2004 ), it could be speculated that in some CI users, poor speech perception is a reflection of reduced cognitive ability to derive meaning from otherwise well-transmitted auditory input (i.e., insufficient higher-level auditory processing ability). To examine this hypothesis, cortical responses to intelligible (i.e., auditory speech) and unintelligible (i.e., auditory non-speech) stimuli can be compared, as done previously in both adult (Pollonini et al, 2014 ; Olds et al, 2016 ; Lawrence et al, 2018 ) and pediatric (Mushtaq et al, 2019 ) listeners using fNIRS. Consistent with findings from these studies, in which greater cortical activation was observed with increased speech intelligibility (Pollonini et al, 2014 ; Olds et al, 2016 ; Lawrence et al, 2018 ), it was hypothesized that intelligible speech (i.e., our normal auditory speech condition) would elicit stronger responses in temporal brain regions of NH listeners, compared with CI users, and that the contrast between cortical responses to the intelligible and unintelligible stimuli will be greatest amongst NH control children.…”

<p>Shedding Light On The Human Auditory Cortex: A Review Of The Advances In Near Infrared Spectroscopy (NIRS)</p>

“…Additionally, fNIRS has a much higher temporal resolution than fMRI, with sampling rates of up to 100Hz compared to fMRI's 0.5 Hz. 36 This allows for both event-related 35,37 and block designs. 38 However, it is important to note that this is still much slower than methods such as EEG that do not rely on sluggish hemodynamic responses, and instead measure more instantaneous electrical pulses.…”

Analysis methods for measuring fNIRS responses generated by a block-design paradigm

“…A primary use has been the investigation of cortical processing of physical qualities of sound, such as intensity, and amplitude and frequency modulations, and auditory-spatial cues (Weder et al, 2020;Weder et al, 2018;Zhang et al, 2018). fNIRS has also been employed to evaluate the perceptual qualities of speech and listening effort, as well as language development in normal-hearing and hearing-impaired populations (Anderson et al, 2019;Lawrence et al, 2018;Mushtaq et al, 2019;Pollonini et al, 2014;Rovetti et al, 2019;Rowland et al, 2018;Sevy et al, 2010;Wiggins et al, 2016b;Wijayasiri et al, 2017;Zhang et al, 2020). Research questions relating to the development of auditory cortical function (Gervain et al, 2008), and cortical reorganization following impaired sensory input and subsequent rehabilitation (Anderson et al, 2017;Wiggins and Hartley, 2015) have been investigated using fNIRS, as have outcomes related to cochlear implantation (Anderson et al, 2019) and auditory pathologies such as tinnitus (Basura et al, 2018;Shoushtarian et al, 2020).…”